The present invention relates to electrical sensing circuitry for a particle-analyzing device the circuitry including circuit elements, and circuit parameters for the circuit elements, which render the output signals from the electrical sensing circuitry independent of the diameter of the aperture in the particle-sensing device and/or independent of the conductivity of the electrolyte in the bodies of liquid in the sensing device on opposite sides of the aperture so that signals generated by particles passing through the aperture are more accurately related to the size or volume of the particle.
The particle-analyzing device with which the teachings of the present invention are intended to be utilized was first disclosed in U.S. Pat. No. 2,656,508. This particle-analyzing device operated on a principle often now referred to as the "COULTER" principle. According to this principle, the passage of a microscopic particle suspended in a conducting liquid through an aperture having dimensions which approximate those of the particles, causes a change in the resistance of the electrical path through the liquid effectively contained in the aperture, if the material of the particle in the liquid had different conductivities. Studies have shown that the magnitude of this change is proportional to the volume of the particle where the cross-sectional area of the particle is smaller than the cross-sectional area of the aperture and the particle is smaller in diameter than the axial length of the aperture. This volume is the volume of actual particulate matter irrespective of the geometric configuration of the particle.
Typically, the particle-analyzing device includes a pair of electrodes each of which is positioned on either side of the aperture. An electrical power source is coupled to the electrodes and typically has had a high output impedance. A voltage sensitive signal-detecting circuit is connected across the electrodes and usually includes an A.C. coupling capacitor (D.C. blocking capacitor) so that the signal-detecting circuit will only sense changes in the voltage across the aperture caused by the passage of a particle through the aperture. These signals are commonly referred to as particle pulses and are channeled from the amplifier to other electrical circuitry for the analysis of the pulse height and for counting the pulses.
Examples of particle-analyzing devices having the structure and associated electrical circuitry described above can be found not only in U.S. Pat. No. 2,656,508 but also in U.S. Pat. Nos. 2,869,078; 2,985,830; 3,015,775; and 3,122,431.
Under proper conditions the particle-analyzing devices described above will generate electrical pulses the respective amplitudes of which are linear functions of the volume of the respective particles passing through the aperture. It is therefore a relatively simple matter to calibrate the analyzing electrical circuitry which receives the particle pulses from the signal detecting circuit. Since the initial effect of the passage of a particle through the aperture is a change of resistance, the obvious method of utilizing this change of resistance was to pass a known constant current through the resistance and detect the change of voltage which resulted. This method was so straightforward that for years no other method was considered. However, the prior art electrical sensing circuits were sensitive to changes in diameter of the aperture and to the conductivity of the liquid medium in which particles were suspended.
The conductivity of the liquid which usually contains an electrolyte is a function of composition temperature and concentration of the electrolyte in the liquid. A change in conductivity resulted in offsetting the calibration of the analyzing circuitry such that a given pulse amplitude would no longer be an accurate indication of the size of the particle generating the pulse. Various electrical sensing circuits have been proposed for providing some compensation for changes in electrolyte conductivity. Examples of these prior art circuits may be found in U.S. Pat. Nos. 3,259,842; and 3,706,030; Canadian Pat. No. 864,075; and Russian Pat. No. 274,474. Hereinafter, other electrical sensing circuits including circuitry for compensating for changes in electrolyte resistivity or conductivity are described in detail.
Even with electrical sensing circuitry which compensated for changes in electrolyte conductivity, the output signals from these circuits were still sensitive to the diameter of the aperture, often referred to as the sensing aperture. Normally, aperture diameter does not vary. However, in highly automated blood cell counters utilizing a particle-analyzing device of the type described above where the blood cell counter is utilized often, a film frequently forms on the inside surface of the aperture thus changing the effective diameter of the aperture resulting in signals which are not accurately related to the size of the blood cell passing through the aperture. This film is sometimes so thin as to be undetectable by microscope. In any event, when unexplained calibration shifts of the calibration settings for the analyzing circuitry occurred, cleaning of the aperture was resorted to in order to obtain the original calibration settings. It has also been noted that in some instances the materials used in fabricating the scanning aperture are slightly hygroscopic. An immersion in an aqueous or other electrolyte of the wafer containing the scanning aperture sometimes causes perceptible swelling of the wafer. This phenomenon also causes a change in the aperture diameter. As will be described hereinafter, the electrical sensing circuitry of the present invention provides, by proper selection of circuit elements and the parameters of the circuit elements, an advantageous and surprising result, namely, the provision of output signals from a particle-analyzing device which are essentially independent of the aperture diameter.
Also, the electrical sensing circuits of the present invention provide additional circuit elements having circuit parameters selected according to the teachings of the invention resulting in output signals which are independent of electrolyte conductivity.
According to the invention, there is provided electrical sensing circuitry for a particle-analyzing device wherein a liquid containing particles is caused to flow through a sensing aperture on either side of which is located a sensing electrode, said circuitry including, means coupled to said electrodes for establishing an electric excitation current through said aperture and means coupled to said electrodes for detecting signals generated by particles passing through said aperture, said means for establishing an electric excitation current through said aperture having a low output impedance and said means coupled to said sensing electrodes for detecting signals generated by particles passing through said aperture having a low input impedance at D.C. and at the frequencies of the particle-generated signals.
Also, according to the invention, there is provided electrical sensing circuitry for a particle-analyzing device wherein a liquid containing particles is caused to flow through a sensing aperture on either side of which is located a sensing electrode, said circuitry including means coupled to said electrodes for establishing an electric excitation current through said aperture, means coupled to said electrodes for detecting signals generated by particles passing through said aperture, and electrical monitoring means coupled to said signal-detecting means for monitoring the conductivity of the liquid containing particles and electrically for altering the output signals from said signal-detecting means relative to changes in liquid conductivity thereby to render said output signals independent of liquid conductivity.